Molecules with binding specificity for the complex oligosaccharide moiety of the fetal alpha-globulin fetuin have been detected on murine thymocytes and splenocytes by fetuin-sepharose affinity chromatography. Plant lectins (saccharide-binding molecules) have proved valuable tools for studying properties of cell membranes, since most external membrane proteins are glycosylated; and mammalian cell lectins such as the fetuin-binding lymphocyte lectin may be important in specific recognition occurring during development or triggering of mature cell function. Because sugars are branched molecules, and because lectins can discriminate very fine differences in structure, a lectin: oligosaccharide recognition system can be very specific, and the literature contains examples of developmental and neoplastic changes in membrane lectins and lectin receptors. Lymphocyte lectins will be isolated from normal murine T and B lymphocytes by affinity chromatography and characterized in terms of physiochemical properties and binding specificity. The presence and amount of lectins on fetal lymphocytes, neoplastic lymphoid cell lines, and nonlymphoid cells will also be determined; and the saccharide specificites of lectins on lymphocytes of different mouse strains will be compared. Specific antibodies to the membrane lectins will be prepared, and the effects of purified lectin or anti-lectin on lymphoid cell function in mitogen or antigen-induced blastogenesis systems will be measured to assess the role of membrane lectins in cellular interactions required for lymphocyte function.